This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/FR99/01280 which has an International filing date of Jun. 1, 1999, which designated the United States of America.
The present invention relates to a method of making car bodywork, the method enabling the tooling used in assembly stations to be as lightweight as possible so as to enable robots to be used as the tools for setting up the bodywork in a welding station and/or the tools for handling bodywork subassemblies that are to be assembled together.
In present assembly lines for assembling car bodywork, the subassemblies are positioned relative to one another and relative to the welding robots by means of tooling that is rigid and heavy. By their very nature, bodywork pieces are deformable when subjected to stresses and the tooling used for making bodywork of determined and reproducible shape performs two functions: firstly it constitutes reinforcement for each deformable subassembly so as to stiffen it, and secondly it constitutes a stable interface enabling accurate positioning to be performed in a fixed frame of reference related to the welding robots.
More precisely, two large families of assembly station are known where motor vehicle bodywork parts are welded together. In the first family, tacked-together bodywork parts are fed into a station and tools hold the bodywork in shape. In the second family, bodywork is brought to a station in the form of subassemblies on frames, and they are held in position either as before by tools belonging to the welding stations, or else by the frames being interlocated relative to one another on a base which is itself referenced in the welding station.
To illustrate the main subassemblies constituting vehicle bodywork, reference can be made to FIG. 1 which shows an underbody 1 forming the bottom structure of the bodywork with an engine compartment 2 at the front which is a box-like structure possessing considerable rigidity.
Another subassembly is constituted by a rear lower panel 3 for fixing to the underbody and to the rear portions of two body sides such as 4 (only one side is shown in FIG. 1), the body sides being also assembled to the sides of the underbody 1. On top, a fourth group of elements comprising a roof panel 5 and two cross-members 6 and 7 extends between the two body sides, and more exactly between the two top portions of the body sides such as 4, which portions are known as xe2x80x9ccant railsxe2x80x9d.
In the first family, the assembly shown is preassembled and reaches the welding station on a base which supports the underbody 1 in referenced manner. The tools for holding the body in shape are then constituted by frames which are moved up to said structure in order to be coupled thereto and which are simultaneously held in identified positions either in an overall structure or else by interlocking.
In the second family, the underbody is brought to the welding station on its own on a base in which its position is referenced, and which is itself placed with precision in the frame of reference of the station, after which the other subassemblies are brought in, each associated with a frame (generally having clamps for securing the subassemblies), which frames are either assembled to the base and positioned relative thereto by means of external tools belonging to a general structure, or else they are mutually interlocked so as to form an undeformable xe2x80x9crigidxe2x80x9d box in which the subassemblies are in the correct positions relative to one another and relative to the welding robot, ready to be welded together.
The major drawback of such welding stations lies in the considerable weight and size of the tooling for holding the bodywork in shape, which tooling firstly obstructs access for the welding robots and secondly, and above all, requires handling means that must necessarily be robust and therefore likewise bulky. At present it is not possible for this handling to be performed by robots since it is known that the maximum load which robots can carry is no greater than 250 kg to 350 kg.
The present invention stems from the observation whereby the greatest difficulty in setting up the shape of motor vehicle bodywork lies at the end of the underbody remote from the engine compartment, i.e. a location where the underbody is itself relatively deformable. In contrast, beside the engine compartment of the underbody, it has been found that it can be helpful, or even sufficient, to take advantage of its stiffness given that the engine compartment is in the form of a box structure that is practically undeformable, at least in association with the forces used to hold the assembled subassemblies in a welding station before or during welding.
The present invention thus provides a method of making car bodywork, the method including a step of assembling body sides to an underbody, the underbody including a rigid engine compartment at one end and being placed in an assembly station in a precise position determined in an orthogonal frame of reference having directions X, Y, and Z, the method consisting in putting the body sides into position and holding them in position in the frame of reference of the station by means of two parallel tools each co-operating with a respective body side, and at least one transverse tool extending transversely to said parallel tools, the three tools being connected to one another and to the X, Y, Z frame of reference of the station in such a manner that the transverse tool is placed and held in a position that is precise and locked in the frame of reference and that it forms a spacer and bracing tool in the Y direction for the two parallel tools in a zone that is remote from the engine compartment of the underbody, the method also consisting in maintaining a force pressing each of the parallel tools at least against the sides of the engine compartment of the underbody while the body sides are being assembled to the underbody.
The transverse tool can be fixed and held in a position that is accurate and locked in the frame of reference of the station in various ways. Thus, the tool can be placed in receiver members belonging to the station (or to the underbody support) so that all of its degrees of freedom are immobilized. The parallel tools do not participate in this holding. In another case, they can participate in holding it in this way, e.g. by being immobilized in the X and Z directions in the frame of reference, the transverse tool being immobilized solely in the Y direction and co-operating with the parallel tools so as to be immobilized in the X and Z directions. Numerous variants can be found for implementing this fixing and holding of the transverse tool without going beyond the ambit of the invention.
The method is applicable to both of the two large families of car bodywork assembly methods. When the two body sides reach the welding station preassembled to the underbody, one way of proceeding could be as follows: the transverse tool is brought up to the preassembled bodywork to take charge of the rear lower panel, if one exists, and then be completely immobilized in the frame of reference of the station; thereafter parallel side tools are brought up to the body sides which they take in charge and stiffen by means of clamps they have for this purpose in conventional manner, and they are then immobilized in the X and Z directions on supports belonging to the station, or for one or other of these directions of immobilization by means of the transverse tool, and in the Y direction by co-operation with the transverse tool via one end while thrust means are implemented towards the other end close to the engine compartment to press the body sides against said compartment. The thrust means can be constituted by a simple actuator extending between the station and each of the side tools while being capable of being retracted at the time said tools dock or are withdrawn.
In some bodywork, there is no rear lower panel; thus the transverse tool serves solely to serve as a bearing spacer and brace for the parallel tools at the rear of the bodywork.
In the second family of assembly stations, the tools constitute supports for the rear lower panel and the body sides, for example, and they are brought into the welding station carrying their subassemblies, and once in the welding station they are put into place, e.g. in the manner mentioned above.
In certain applications, in the event of the stiffness of the engine compartment being judged to be insufficient, or in order to avoid excessive compression of the engine compartment between the side tools it can be advantageous to provide for the two side tools to be fitted near their ends close to the engine compartment with mutual bearing elements that come into contact with one another under the effect of Y direction thrust to which they are subjected. These bearing elements can include locked connection means capable of relieving the thrust actuators from the effect of any forces on the bodywork tending to move the body sides apart from each other.